Silurian Period 444 to 416 mya

Summary

The Silurian Period was named after a Welsh
tribe. Gondwana continued to drift across the South Pole. After the recent mass
extinction, corals grew rapidly in the warm shallow seas. The first plants and
animals began to invade the land.

Continents

As the Silurian Period began, Gondwana
continued its travel across the South Pole and a glaciation affected many
continents. This had ended by the mid-Silurian when the global climate become
much warmer, especially near the equator where dry conditions led to extensive
salt deposits. But glaciers remained at the poles. Globally, sea levels rose.

At the end of the Silurian, several smaller
continents collided to form Euramerica, a new
supercontinent. These collisions also created new mountain ranges.

In the animal world the major event was the
appearance of fish with jaws and bones.

A lichen ("ly-ken" or "li-chen") is a
combination of a fungus and either a blue-green bacteria or green algae which lives inside
it. The fungus gets food from the algae or bacteria and they get a safe home
from the fungus. This is an example of symbiosis.

Lichens might have been the first things to
live on land. Today they invade new islands made by volcanoes before any other
form of life.

Early in their history, plants could only
grow in water. There were many benefits waiting for any plant which could grow
up out of the water. It could get more sunlight, and so more energy, than its algal cousins who were trapped in the sea. A plant which could leave the
water and live on land would escape from animals which ate it and find more
room to grow.

But there was a price to be paid for these
benefits. The plant had to evolve ways to make itself stiff enough to stick out
of the water. It had to stop itself drying out and somehow it still had to get
the water and minerals which it needed to grow. It also had to take the food it
made from sunlight down to the rest of the plant.

About 470 mya the first plants to evolve
these features began to grow out of the water.

Bony fish

Bony fish evolved jaws about 50 million
years later, developed heavy brain casing about 10 million years after this,
and split into two main groups of fish: the lobe finned and the ray finned. At
about the same time the shark group first
appeared.

Invertebrates onto Land

Animals need plants for food, since only
plants can capture the Sun's energy. Once the ferns had moved onto the land, it wasn't long before animals followed
them. The first were the arthropods.

Cockroaches, dragonflies and scorpions
began to live on land around 450 mya. They had to solve the same sort of
problems as the plants did when they left the water. They already had stiff
skins and by evolving a waxy coat they could stop themselves drying out.

A bigger problem was how to get rid of
their waste products without having lots of water around them to wash the waste
away. They managed to solve this too.

Female dragonfly laying eggs. Image
courtesy of Andreas Trepte

But most difficult of all was how to reproduce without water. This was so difficult that the first arthropods on
land, like the ferns before them, were still tied to the water for reproduction.
Dragonflies need water to reproduce even today.